In the technical field of conventional vehicle headlights, there is a certain demand for providing a vehicle headlight to project light with higher luminance in order to allow for operation of the vehicle during nighttime driving just like during daytime driving. In response to such a demand, there have been proposed various headlights, such as, those employing a high luminous flux light source including halogen lamps, HID lamps, and the like, those with improved optical systems, and the like in order to improve the luminance (brightness, luminous flux, light emission efficiency and the like). Such a vehicle headlight is disclosed in Japanese Patent Application Laid-Open No. 2007-59162 or U.S. Patent Application No. 2007/047250A1 corresponding thereto, for example.
In general, human eyes have characteristics such that the sensitivity of eyes under dark environment (e.g., during nighttime driving) increases more to the red light than to the blue light. In consideration of these characteristics, Japanese Patent Application Laid-Open No. 2008-204727 proposes a vehicle headlight for that purpose. As shown in FIGS. 1A and 1B, this vehicle headlight can illuminate the front area A1 with light having a larger amount of the blue light component than the red light component in order to enhance the visibility during nighttime driving and also can illuminate the central area A2 in the front area A1 with light having a larger amount of the red light component than the other light components in order to enhance the recognition by a driver with respect to color, shape, or other features of the road or an object on the road (as well as the area A3 above the horizontal line in the distribution diagram).
However, it has not been conventionally known how the blue light affects the awareness with the peripheral vision under dark environment (e.g., during nighttime driving).
FIG. 2A is an explanatory diagram illustrating the central vision and the peripheral vision of a driver, and FIG. 2B is an explanatory diagram illustrating the relationship between the central vision, the peripheral vision, the cone cell, and the rod cell of a driver. Furthermore, FIG. 3 is a flow chart describing the flow of how a driver can recognize an object (such as a pedestrian and an obstacle) existing in the peripheral visual field.
Now examine how the driver who keeps close watch on a farther area (see, for example, three circles in FIG. 2A and the center arrowed portion in FIG. 2B) can recognize an object (such as a pedestrian and an obstacle) existing in the peripheral visual field. In this case, as shown in FIG. 3, the driver first becomes aware of the object by his/her peripheral vision (with the use of rod cells). (Step S1: Yes) Then, the driver directs his/her eyes to the direction where the object is located (step S2). After that, the driver can recognize the object such as the color and shape thereof by his/her central vision (with the use of cone cells). (Step S3) If the driver does not become aware of the object by his/her peripheral vision (with the use of rod cells), this means that the driver has missed the object (step S4). Namely, it is important for a driver to first become aware of an object that exists in the peripheral visual field. If the driver does not become aware of the object as it exists in the peripheral visual field, he/she may never recognize the object.
In particular, under dark environment (e.g., during nighttime driving), there are many situations in which the awareness with the peripheral vision (equal to the use of rod cells, meaning the scotopic sensitivity) is required or helpful, such as during right or left turns at an intersection, bifurcation, changing lanes, and keeping aligned in a lane. Therefore, it is important to cause a driver to become aware of such a situation earlier. For example, since the area closer to the front side of the vehicle body when viewed from a driver side is not sufficiently illuminated with light from a vehicle headlight, it is difficult for a driver to become aware of an object existing in the peripheral visual field. In addition, the wider the road width is, the more difficult it is for a driver to become aware of an object closer to the vehicle front side.
In general, cone cells and rod cells are distributed over the retina of human eyes. FIG. 4 is a table listing the comparisons between the peripheral vision and the central vision. As shown in the table of FIG. 4, the cone cells and the rod cells are very different from each other in terms of the distributed area, the number thereof, the function, the role, the active environment, and the like. The rod cells are cells for detecting an object on which a driver's eyes is to be turned, and are distributed around the field of view (peripheral vision). The rod cells can work under dark environment (scotopic vision). On the other hand, the cone cells are cells for identifying and recognizing an object while obtaining and determining detailed information, and are distributed over the central area of the field of view (central vision). The cone cells can work under the bright environment (photopic vision). Specifically, human eyes can sense light from a bright area to a dark area by the complementary effect of both the photoreceptor cells (rod and cone cells).
Unlike daytime driving, nighttime driving is performed under dark environment (meaning that the photopic vision is not mainly utilized). Since the road is illuminated with a headlight to a certain degree, it is not a completely dark environment (meaning that the scotopic vision may not be mainly utilized). Namely, the environment during nighttime driving is a dim environment with the use of mesopic vision between the photopic vision and the scotopic vision (meaning that both the cone and rod cells are activated). In this case, the adaptation illuminance is approximately 1 lx.
FIG. 5 is an explanatory graph showing the relative luminosity factor V(λ) in the photopic vision and the relative luminosity factor V′(λ) in the scotopic vision. As shown, the peak of the luminosity curve is shifted to the short wavelength side while the photopic vision is shifted via the mesopic vision to the scotopic vision. This peak shift is derived from the difference between the spectral sensitivities of the cone cells and the rod cells.
The present inventors have conducted intensive studies on the visual features of human eyes, and considered that the enhanced energy components with shorter wavelengths (bluish light component) could effectively stimulate the rod cells under dark environment (e.g., during nighttime driving), thereby facilitating awareness with the peripheral vision.
Based on this assumption, the inventors have performed various experiments and conducted studies based thereon, and found that the increased amount of energy components with shorter wavelengths (bluish light component) can facilitate an earlier awareness with the peripheral vision under dark environment (e.g., during nighttime driving) (with shorter reaction speed while lowering the missing-out rate), thereby resulting in the presently disclosed subject matter.